Yol. 59.] NITROGEN AND CARBON IN CLAYS AND MARLS. 135 



of soluble nitrogen did not vary much in the two classes of soils, 

 the amounts of soluble humus, 1 and consequently the percentage 

 of nitrogen in the humus, varied considerably. 



Table III. — Soluble Humus and Soluble Nitrogen in Arid and Humid 

 Soils in California. 



1 



1 



Soluble 

 1 humus. 



Nitrogen in 

 soluble 

 humus. 



Soluble 



Nitrogen 



in soil. 



Per cent. 



o-ioi 



0-102 

 0132 



Arid soils : uplands, California 



Arid soils: lowlands, California ... 

 Humid soils 



Per cent. 

 0-75 

 0-99 

 304 



Per cent. 



15-87 



10-03 



5-24 





The total nitrogen and the organic carbon in the soils were not 

 determined, but the results tabulated above afford clear evidence of 

 the essential difference in the character of the humus produced 

 under the different climatic conditions. Hilgard also showed, in 

 accordance with the observations of Armsby, Wollny, and others, 

 that accumulation of nitrogen is promoted by the presence of earthy 

 carbonates, and that ferric hydroxide acts in the opposite direction. 



Reference has already been made to the decreasing rate of the 

 decomposition of organic residues, as a necessary consequence of the 

 disappearance of their less stable constituents. It is probable 

 that almost the whole of the organic matter 2 which remains 

 near the surface will, sooner or later, be resolved into substances 

 which' living vegetation is able to assimilate. It must, however, 

 be borne in mind that nearly all soils contain, in addition to the 

 residues of their present and past vegetation, more or less organic 

 matter belonging to the original deposit, and that this organic 

 matter has, in many cases, according to the geological formation 

 to which it belongs, undergone further changes which render the 

 production, or re-production, of anything of the nature of humus 

 impossible. 



It is evident therefore, that if, as is undoubtedly the case, it is, 

 useful to make a distinction between the soluble, or more im- 

 mediately available, and the insoluble humus, it is equally essential 



1 Soluble humus (G-randeau's matiere noire) is the substance dissolved 

 from soils by weak alkali-solutions, after removal of the bases by extracting 

 the soil with dilute hydrochloric acid. The term ' soluble nitrogen ' refers 

 to the organic nitrogen present in the soluble humus obtained in the manner 

 described. 



2 Except, of course, certain animal remains, especially the hard chitinous 

 portions of insects which are extremely resistant, and which, as pointed out by 

 P. E. Miiller ('Die Naturlichen Humnsformen ' Berlin, 1887) may sometimes 

 occur in considerable quantity. 



